The present invention relates in general to an optical scanner such as a printhead, and in particular to an optical scanner including a memory device that stores operational characteristics of the optical scanner and/or of a corresponding electrophotographic device to which the optical scanner is installed. The present invention is also related to test fixtures for deriving such operational characteristics.
In electrophotography, a latent image is created on the surface of an electrostatically charged photoconductive drum by exposing select portions of the drum surface to laser light. Essentially, the density of the electrostatic charge on the surface of the drum is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the surface of the drum to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the drum surface in a manner that corresponds to the electrostatic density altered by the laser beam. Subsequently, a print medium, such as paper, is given an electrostatic charge opposite that of the toner and is pressed against the drum surface. As the medium passes the drum, the toner is pulled onto the surface of the medium in a pattern corresponding to the latent image written to the drum surface. The medium then passes through a fuser that applies heat and pressure to the toner on the medium. The heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the fuser pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
In order to produce an accurate representation of an image to be printed, it is necessary for the printhead laser(s) to write to the drum in a scan direction, which is defined by a straight line that is perpendicular to the direction of movement of print media relative to the drum (the process direction). However, a scanning laser beam may follow a scan path that is not perpendicular to the process direction. Should the scan path of a laser beam deviate from the ideal scan direction, print artifacts may result. Laser beam scan path deviation is further complicated in color devices because excessive color to color mis-registration may cause color variation among other print artifacts.
Moreover, each laser should be capable of writing a line of evenly spaced print elements (Pels) on the surface of the drum. However, manufacturing tolerances, imperfections of optical devices in the optical system, and the inherent configuration of the printhead may cause variations in the spacing between written Pels along a scan line, which is referred to herein as scan line nonlinearity. Particularly, the velocity of the laser beam may vary across the scan line, which typically causes consecutive Pels to be written farther apart near the end portions of the scan line, and closer together near the middle portion of the scan line.
Still further, during a printing operation, the amount of toner attracted to the drum surface is highly sensitive to the amount of optical energy applied to the drum. Thus, the overall print quality is sensitive to the energy output by the printhead lasers(s). The amount of optical energy required on the surface of the drum to achieve a predetermined overall print quality may be determined by a number of factors including the print quality settings and print resolution of the electrophotographic device. Also, fundamental characteristics of the laser(s) in the printhead, such as laser turn on current and laser beam efficiency, must be known. Determining such fundamental laser characteristics typically requires meticulous factory calibration and relatively tight tolerances in the optical components of the printhead, resulting in an increased time and cost to manufacture the corresponding electrophotographic device.